Photocathode on bveled end plate of electron tube



B. H. VINE March 29, 1966 PHOTOCATHODE ON BEVEL ED END PLATE 0F ELECTRON TUBE Filed Dec. 21, 1962 2 Sheets-Sheet l INVENTOR. BENJAMIN H. V/NE B. H. VINE March 29, 1966 PHOTOCATHODE 0N BEVELED END PLATE OF ELECTRON TUBE Filed D80. 21, 1962 2 Sheets-Sheet 2 INVENTOR BiA/JAM/A/ 1% WM? ArToxA A'r United States Patent O The present invention relates to electron tubes having photocathodes and particularly to an improved method of applying a photoemissive coating to a substrate and sealing the substrate across an opening in an electron tube envelope.

One type of photocathode employed widely is a semitransparent type. This typeincludes a transparent substrate, made of glass for example, and having thereon a coating that responds in electron emission to light impinging thereon. This type of cathode finds utility in a wide variety of conversion type electron tubes such as camera tubes, image tubes, photo-multipliers, and others.

One form of photocathode comprises a silver-bismuth layer on a transparent substrate. The silver-bismuth layer is suitably activated by exposure to cesium vapor. The invention may also be practiced with other forms of photocathodes, such as the multi-alkali type, for example.

Photocathodes of the type referred to are destroyed by exposure to air. For this reason, the coatings thereof are applied, as by evaporating techniques, in an evacuated processing chamber in which the two operaitons of applying the coatings to a substrate and sealing the coated substrate across an opening in an electron tube envelope, are performed.

Where the coating applications are effected in the evacuated envelope of an electron tube, it is accompanied by an objectionable contamination of components within the tube other than the substrate being coated. Such contamination is particularly severe when evaporating a layer of cesium onto the substrate. Such severe contamination is a consequence of a technique found desirable in applying the cesium to a substrate. This technique requires an indirect diffusion from an applicator to the substrate. Such indirect difiusion is usually effected by employing electrode elements within the tube as bafiles, disposed across a direct path between the applicator and substrate. In this situation, some deposition of the cesium on the electrode elements referred to is unavoidable. Such undesirable deposition of cesium is difficult to remove and may contribute to faulty operation of the tube because of the relatively low work function of cesium.

While application of the coatings in a separate evacuated processing chamber, other than the envelope of an electron tube, avoids contamination of electrode elements in a tube by the coating materials, a resort to this type of processing is accompanied by a problem of conveniently sealing the coated substrate across an opening in the tube envelope, at temperatures sufficiently low to prevent harm to the applied coatings. Accordingly, it is an object of the invention to provide an improved electron tube having a photocathode.

It is a further object to provide an improved method of fabricating an electron tube having a photocathode.

Another object is to provide a method involving the application of a photoemissive coating to a substrate at a location remote from a tube envelope with which the coated substrate eventually is to be incorporated, to preserve electrode elements within the tube envelope from contamination by one or more of the coating materials.

A further object is to provide a method wherein a substrate is coated with photoemissive materials in one location within an evacuated ambient and the coated substrate is sealed in an improved manner at another location within the ambient referred to.

"ice

Another object is to provide a more advantageous method of sealing across an opening in an electron tube envelope, a substrate coated with materials which must not be exposed to air.

Briefly considered, one example of the invention involves evaporating photoemissive coating material on a glass substrate at one location within an evacuated bell jar, and then moving the coated substrate to a position wherein it covers an opening in an electron tube envelope at another location within the bell jar. The tube envelope has a structure which contributes to the formation of an annular groove between the envelope and substrate when the latter is positioned over the opening referred to. The surfaces defining the groove are metallized to facilitate a wetting thereof by a molten sealing material to be introduced into the groove. The sealing material, which may be indium or a tin-indium eutectic, is directed into the region of the groove wherein it is suitably confined by a mold. Applicant has found that the indium does not wet an uncoated portion adjacent the groove, and hence is restrained from migrating to a location within the tube envelope, even in the presence of a pressure exerted on the molten indium suflicient to cause it to migrate fully around the groove. Indium is particularly advantageous for use as the sealing material since its melting point temperature is sufficiently low to prevent harm to the photoemissive coatings applied to the substrate, asby liberation of gases within the tube. Furthermore, indium has a low vapor pressure even at the processing temperatures within the bell jar.

The bell jar may have a sufiiciently low pressure to provide the required vacuum condition within the sealed tube envelope. However, if a lower vacuum is desired, the tube envelope may be provided with an exhaust tubulation for further evacuating the envelope.

Further objects and features of the invention will become evident as the present description continues.

In the drawing:

FIG. 1 is an elevational view, partly in section, of a pickup tube incorporating the invention;

FIG. 2 is an enlarged fragmentary sectional view of a portion of the tube shown in FIG. 1 and depicts an important feature of the invention;

FIG, 3 shows schematically apparatus that may be employed in practicing the method of the invention;

FIG. 4 is an enlarged structural view, partly in crosssection, of a portion of the apparatus depicted in FIG. 3;

FIG. 5 is a plan view, partly in section, showing structural features of the evaporator tank or chamber shown schematically in FIG. 3; and

FIG. 6 is a sectional view taken along the line 66 of FIG. 5.

The electron tube 10 shown in FIG. 1 presents, by way of example, one embodiment of the invention. This tube is an image orthicon type and except for the photocathode structure 12 may be a type structurally described in US. Patent No. 2,942,132, issued June 21, 1960 to Rotow et al., or U.S. Patent No. 2,433,741, issued January 6, 1948 to Weimer.

In the example shown in FIGS. 1 and 2, the photocathode structure 12 includes a substrate 14 having on the inner face thereof a coating or layer 16 consisting of an alloy of silver and bismuth, activated by cesium 18. The coatings 16 and 18 are spaced inwardly from the edge of the substrate 14. The substrate 14 also includes an annular metallic coating 20 extending inwardly from the edge of substrate 14 to a region wherein it is overlapped by the silver-bismuth coating 16.

The coating 20 serves two important functions in the structure of tube 10. One function is to provide an electrically conductive path between the silver-bismuth coating and a lead 22 connected to a lead-in conductor 24 of tube 10. The other function is to effectively engage a sealing material 26, which may be indium or an alloy thereof such as tin-indium eutectic. To provide a confining region for the sealing material 26 the upper end of envelope portion 28 is provided with an outer annular beveled portion 30 having a metallized coating 32 thereon. The metallic coatings 20 and 32 may consist of a first layer of chromium and a graded layer of chromium and gold thereover.

The upper end of envelope portion 28 also includes an inner annular region 29 lying in a plane normal to the axis of tube 10. This region is free of coating material, which is of advantage in fabricating the tube, as will be made clear in the following description of method features of the invention. The substrate 14 is provided with an edge groove 34 having an advantage in fabrication of the tube.

In a particular tube constructed as shown in FIG. 1,

the substrate 14 had a diameter of three inches, and the diameter of the area thereof having the coatings 16, 18 was two and one quarter inches. The annular portion of the substrate 14 free of coatings 16, 18, had a radial extent of three-eighths of an inch. The coating 20 extended inwardly from the edge of the substrate 14 about one-half inch, to provide a desired overlap with respect to coating 16.

In a tube of the type discussed it is desirable that the coating 16 be uniformly thick and that the electrode elements within the tube 10 be free from contamination by the activating material 18. These desirable ends have not been achieved fully in the past in view of a generally practiced technique of applying the coatings from sources within the tube envelope, and after the tube had been evacuated. This restriction in the location of the coating sources was imposed because of a characteristic of the coating materials which render them destructively sensitive to air. All photocathode materials useful for visible light are completely destroyed on exposure to air or most other gases.

Apparatus for carrying out a novel and advantageous method of applying a photoemissive coating on a substrate at a location remote from the eventual tube envelope and sealing the substrate across the opening in the envelope is shown schematically in FIG. 3. The apparatus includes a bell jar 36 which is suitably evacuated through an exhaust duct 38 to a pressure of about 10 millimeters of mercury and is adapted to be placed in an oven, not shown, for heating the interior thereof to a desired temperature. Within the bell jar 36 and afiixed to the base 40 thereof is a post 42 supporting a bracket 44 which in turn supports a tank 46 containing coating sources. The interior of tank 46 is connected by metal tube 54 having a closed lower end, to a trap 56 cooled by liquid nitrogen 57. The bracket 44 also supports an image orthicon tube portion 48 having an opening 50 for a photocathode structure 12. Also engaged by the bracket 44 are a reservoir 52 of sealing material such as indium or a tin-indium eutectic, and a mold 53.

The bell jar 36 not only has utility in the method described, of forming coatings on the substrate 14 and sealing the substrate across the end of tube envelope portion 28, but also is employed in a bake-out operation to which the tube 10 is subjected prior to deposition of the coatings on the substrate 14. This bake-out is effected by disposing the bell jar in an oven having a temperature of about 400 C. This temperature is sufficiently high to drive out gases occluded in the components of the tube other than the substrate 14.

After the bake-out operation described in the foregoing and prior to the sealing of the coated substrate 14 across the opening in envelope portion 28, the bell jar 36 is returned to room temperature. At room temperature the thermionic cathode of the gun structure (not shown) of the tube 10, is activated by locally heating the cathode 4 to activating temperature, and the coating 16 is applied and activated with cesium 18 b'y a method involving the use of apparatus shown in FIG. 3.

Coating the photoemissive cathode The apparatus for applying to the substrate 14 the photoemissive coatings 16, 18 (FIG. 1), which in the example described comprise a silver-bismuth alloy activated with cesium, serves to apply the coatings at a region within the bell jar remote from the envelope 48, thereby preserving the interior of the envelope from contamination by the coating materials used. Such preservation from contamination is particularly desirable in connection with cesium, which requires for proper application a general diffusion thereof throughout the envelope 28. Such general diffusion necessarily involves some contamination of electrodes within the envelope.

The apparatus, considered in more detail, includes the tank 46 in which sources of silver-bismuth alloy, and cesium after generation from cesium chromate and silicon are adapted to be vaporized upon the substrate 14. The upper end of the tank 46 as viewedin FIG. 3 is open and adapted to be closed by the substrate 14. To effect such closure the substrate 14 is supported by a spider 82 having three arms, two of which 76, 78 are shown. The arms include fingers 80 adapted to enter the annular groove 34 in the edge of substrate 14 for suitably holding the substrate. Wires 74 are suitably wrapped around each arm 76 and a frame 84 of the spider to cause the arms to firmly engage the substrate 14.

A shaft 86 is connected to the frame 84 and extendsthrough a bellows 92 in the base 40 of the bell jar 36, and terminates in a handle 88 manually engageable outside of the bell jar. This facilitates the application of angular movements to the shaft 86. The bellows 92 serves to prevent loss of vacuum within the bell jar.

It will be noted that the tank 46 is positioned at an angle with respect to the axis of the bell jar 36, while the tube envelope 28 is disposed in axially parallel relation with respect to the bell jar. Such positioning permits the spider 82 to place the substrate 14 in coaxial relation with each of the tank 46 and the envelope portion 28 in response to angular movements of the shaft 86.

Within the tank 46 are two vapor sources comprising a bismuth-silver evaporator 96 and a cesium vapor generator 98, as shown in FIGS. 5 and 6. The bismuth-silver evaporator 96 comprises an annular trough or shield 100 formed at the lower end of a tubular metal member 102. The upper end of member 102 engages and is supported by the side wall of trough 46 by means of a flange 103. Within the trough 100 is disposed a wire 104 made of molybdenum, for example, and having several pellets of silver-bismuth alloy fused therearound, four pellets 106 being shown in FIG. 5. The pellets are spaced from the walls of the shield 100 to prevent heat loss from the pellets. The ends of the wire 104 are connected to terminals 108, 110 for suitable local electrical energization to a temperature at which the silver-bismuth alloy pellets 106, vaporize. This temperature is about 1000 C. The vapors of silver and bismuth thus produced travel upwardly and form coating 16 (FIG. 1) on the substrate 14 positioned thereover, as shown in FIG. 3.

The thickness of the silver-bismuth coating 16 is determined by the measured light transmission through the substrate having this coating. Thus when the light transmission through the coated substrate is reduced to about 50% of its initial value, desired coating thickness is achieved.

The cesium generator 98 comprises a trough containing a material 112, consisting of cesium chromate and silicon, and supported on the outer wall of metal member 102 in spaced relation therewith to prevent heat loss from the trough. The generator 98 may comprise two or more angularly spaced troughs 114, 116 shown in FIG. 5. Each trough may have individual leads to a suitable current supply or the troughs may be connected in series by means of a conductor 118 (FIG. 5) to terminals 120, 122. The current supply to which the terminals 120, 122'are adapted to be connected should be sufiicient to locally heat the troughs 114, 116 and the cesium chromate and silicon 112 therein, to a temperature of 800 C. at which the cesium chromate is reduced to cesium metal. One current supply found suitable for this purpose was one of from 4.5 to 6.5 amperes.

The amount of cesium deposited from the generator 98 is determined by measuring the sensitivity of the photocathode produced by the coating 16, 18 on the substrate 14. When peak sensitivity is obtained, further heating of the cesium generator 98 is stopped.

The mounting of the cesium generator 98 outside the tubular metal member 102 provides the desired diffusion of vaporized cesium, generally throughout the interior of the tank 46. Thus the flange 103 blocks direct travel of the cesium vapor from the generator 98 to a substrate closing the opening 124 of the tank. Instead, the structure shown in FIG. 6 requires the vaporized cesium from the generator 98 to travel downwardly in the annular space between the member 102 and the side wall of tank 46, and then upwardly through the lower opening 126 in the member 102 and onto a substrate closing the opening 124. This generalized diffusion of the cesium vapors within the tank 46 is desirable for the application of a satisfactory coating on the substrate. While the substrate to be coated is not shown in FIG. 6, in the interests of clarity, it is shown at 14 in FIG. 3 closing the upper open end of the tank 46, during a coating application.

' Any vaporized coating material remaining in the tank 46 after the coating applications referred to have been terminated is condensed in the lower end portion of tube 56 immersed in liquid nitrogen.

After application of the bismuth-silver coating 16 and the activation thereof with cesium 18, the shaft 86 is manually operated to transfer the coated substrate 14 into position over the open end of tube envelope portion 28 (FIG. 3) and to seat the substrate on the uncoated land 29 (FIG. 2) of the upper end of this portion. In this position, the edge of the substrate and the upper end of the envelope 48 define the annular space 70 (FIG. 4) confined by the mold 53, in readiness for the indium casting step to be described.

. Casting of sealing material Considering the sealing of the coated substrate 14 across the opening in the envelope portion 28, it will be'noted in FIG. 3 that the reservoir 52 comprises an elongated tube 56 having an open end and made of a material such as glass. Within the tube 56 is a mass 58 of indium or tin-indium eutectic which, at the processing temperature of 165 C. within the bell jar 36, is in a molten state but has a low vapor pressure. This processing temperature of 165 C. is secured by disposing the bell jar 36 in an oven (not shown) having this temperature. The indium, having a melting point of 155 C. is fluid at this temperature. A tin-indium eutectic has a melting point temperature of 118 C. A plunger 60 is connected to a shaft 62 movably extending through the base 40 of the bell jar, and connected to a manually engageable handle 64. A bellows 66 permis the application of vertical movements to the shaft 62 as viewed in FIG. 3, without loss of vacuum within the bell jar 36. Downward movement of the shaft 62 will cause the plunger 60 to extend into and displace the molten mass 58 of indium or tin-indium eutectic. Such displacement will cause the molten mass 58 to rise and flow through a spout 68 into a region confined by the mold 53. This region includes the annular recess 70 shown in FIG. 4, defined by the substrate 14 and the edge portion 30 of the envelope wall 28 shown in FIG. 2. When the molten indium thus dispensed in the annular recess 70 hardens on subsequent cooling, it forms an effective seal between the substrate 14 and the tube envelope 28.

In order to avoid a bonding of the indium to the mold 53, the mold is preferably made of stainless steel. Freedom from bonding of the indium to a stainless steel mold is believed to be due to the presence of chromium oxide on the surface of the mold. Chromium-containing alloys other than stainless steel are also suitable as the material of the mold.

Applicant has found that molten indium of tin-indium eutectic does not easily wet the metalized coatings 20 and 3% on the surfaces defining the region 20 and is not aided by capillary action in entering the region 70. Therefore, to ensure a migration of the molten material fully into all portions of the region around the tube envelope it is necessary to apply the material with some pressure. This pressure is secured by positioning the dispensing tube 56 in such position that the entrance 72 to spout 68 (FIG. 4) is disposed from about A to inch above the region 70. This provides a head resulting in sufiicient pressure on the indium in the region 70 to urge it into full entrance into all portions of region 70. This pressure is not sufiicient, however, to force the molten indium between the uncoated land 29 (FIG. 2) of the upper edge of envelope 28, and the coating 20 on the substrate 14. Applicant has found that the uncoated portion 29 offers appreciably greater opposition to the flow of the molten indium or tin-indium eutectic than the surfaces having the coatings 20, 30. As a consequence, the region between the land 29 and the substrate 14 is substantially free of indium at the pressure indicated, thereby assuring absence of this material from the interior of the envelope of tube 10 (FIG. 1) where it may be harmful to tube operation.

This manner of casting the indium into the space 70 assures a desired vacuum tight engagement of the cast body 26 (FIG. 2), with the substrate 14 and the tube envelope portion 28.

The particular way in which the indium is dispensed by the reservoir 52 is of advantage with respect to the presence of oxides in the cast body 26. Any oxides that form in the reservoir have been found by applicant to remain on the walls of the container 56 in a form of a skin. During a lowering of the plunger 60, a metal, free from oxide, is dispensed to the spout 68 and to the space 70. Sufiicient indium may be placed in the container 56, so that at the end of a dispensing operation the portion of the indium contaminated by oxides remains in the container.

While indium has been referred to in the foregoing as the casting material for effecting a seal between the substrate 14 and tube envelope portion 28, other materials may be used that have a sufiiciently low melting point so as not to require processing at temperatures at which gases are released within the tube, and which have a sufficiently low vapor pressure at the processing temperature to avoid evaporation thereof. In addition, the casting material should be such that it does not wet the uncoated land 29 of the tube envelope portion 28.

I claim:

1. An electron tube having:

(a) an elongated tubular envelope,

(b) a plate closing one end of said envelope,

(c) said one end including a beveled first surface portion and a second surface portion lying in a plane normal to the longitude axis of said envelope,

(d) said beveled first portion and an annular portion of said plate co-extensive with said beveled portion having metallic coatings thereon,

(e) said second portion being free from metallic coating bonded thereto, and

(f) a cast-body of sealing material engaging only the metallic coatings on said beveled first portion and said annular portion for hermetically sealing said plate to said one end.

2. An electron tube having:

(a) an elongated tubular envelope,

(b) a plate closing one end of said envelope,

(c) said one end having a surface including a beveled first portion and a second portion lying in a plane normal to the longitudinal axis of said envelope,

(d) said beveled first portion and an annular portion of said plate coextensive with said beveled portion having metallic coatings thereon, wettable by a molten sealing material, and defining a recess,

(e) said second portion being free from metallic coating, and

(f) a cast body of said sealing material engaging the metallic coatings on said beveled first portion and said annular portion and substantially filling said recess for hermetically sealing said plate to said one end,

(g) said second portion being free from said sealing material.

3. An electron tube having:

(a) an elongated tubular envelope,

(b) a plate closing one end of said envelope,

(c) said one end having a recessed surface portion extending to the periphery of said envelope, and a second portion lying in a plane normal to the longitude axis of said envelope,

(d) said recessed portion and an annular portion of said plate co-extensive with said beveled portion having metallic coatings thereon, and defining an annular cavity,

(e) said second portion being free from metallic coating, and

(f) a cast body of sealing material engaging only the metallic coatings on said beveled first portion and said annular portion and substantially filling said cavity for hermetically sealing said plate to said one end.

4. An electron tube having:

(a) an elongated tubular glass envelope,

(b) a glass plate closing one end of said envelope,

(c) said one end including a raised portion engaging said plate and a second portion defining one Wall of an annular recess,

(d) said second portion and an annular portion of said plate co-extensive With said second portion having metallic coatings thereon wettable by molten indium,

(e) said raised portion being free from metallic coating, and

(f) a cast body of indium engaging only the metallic coatings on said second portion and said annular portion for hermetically sealing said plate to said one end.

5. A photo-emissive electron tube comprising:

(a) an elongated glass envelope,

(b) a glass plate closing one end of said envelope,

(c) said glass plate having an annular area on the inner surface thereof extending to the periphery of the plate, said area having a metallic coating thereon that is electrically conductive and wettable by molten indium,

(d) said inner surface having a central area overlapping said annular area,

(e) said central area having thereon a photo-emissive coating,

(f) said envelope having an annular recess in the outer wall thereof adjacent to said one end and exposing a portion of said coated annular area,

(g) the surface of said recess having thereon a metallic coating wettable by molten indium, and

(h) a cast body of indium in said recess hermetically sealed to the metallic coatings on the surfaces of said recess and said exposed annular area.

6. A photo-emissive electron tube comprising:

(a) an elongated glass envelope,

(b) a glass plate closing one end of said envelope,

(c) said glass plate having an annular area on the inner surface thereof extending to the periphery of the plate, said area having a metallic coating thereon that is electrically conductive and wettable by a molten sealing material,

(d) said inner surface having a central area partly only, overlapping said annular area,

(e) said central area having thereon a photo-emissive coating,

(f) said envelope having an annular bevel in the outer Wall thereof adjacent to said one end and exposing a portion of said coated annular area,

(g) the surface of said bevel having thereon a metallic coating wettable by said molten sealing material, and

(h) a cast body of said sealing material in said recess hermetically sealed to the metallic coatings on the surface of said bevel and said exposed annular area.

7. A photo-emissive electron tube comprising:

(a) an elongated glass envelope,

(b) a glass plate closing one end of said envelope,

(c) said glass plate having an annular area on the inner surface thereof extending to the periphery of the plate, said area having a metallic coating thereon that is electrically conductive and wettable by material selected from the group consisting of indium and indium alloys,

(d) said inner surface having a central area overlapping only partly, said annular area,

(c) said central area having thereon on all portions thereof a photo-emissive coating,

(f) said envelope having an annular recess in the outer Wall thereof adjacent to said one end and exposing a portion of said coated annular area,

(g) the surface of said recess having thereon a metallic coating wettable by said material,

(h) said recess surface and said portion of said annular area defining a cavity, and

(i) a cast body of said material in said cavity and hermetically sealed to the metallic coatings on the surface of said recess and said exposed annular area.

8. A photo-emissive electron tube comprising:

(a) an elongated glass envelope,

('b) a glass plate closing one end of said envelope,

(0) said glass plate having an inner annular surface portion, said surface portion having a metallic coating that is electrically conductive and wettable by a molten material selected from the group consisting of indium and indium alloys,

((1) the inner surface of said plate having a central surface portion overlapping a portion only, of said annular surface portion,

(e) said central surface portion having thereon a photo-emissive coating,

(f) said envelope having an annular recess extending partly inwardly of the outer Wall thereof adjacent to said one end and exposing a portion of said coated annular surface portion,

(g) the surface of said recess having thereon a metallic coating wettable by said molten material, and

(h) a hardened cast body of said material in said recess hermetically sealed to the metallic coatings on the surface of said recess and said exposed annular surface portion.

References Cited by the Examiner UNITED STATES PATENTS 2,903,319 9/1959 Kuryla et al. 316l8 3,073,981 1/1963 Miller et al 313-65 FOREIGN PATENTS 1,141,321 12/1962 Germany.

GEORGE N. WESTBY, Primary Examiner.

R. SEGAL, Assistant Examiner. 

1. AN ELECTRON TUBE HAVING: (A) AN ELONGATED TUBULAR ENVELOPE, (B) A PLATE CLOSING ONE END OF SAID ENVELOPE, (C) SAID ONE END INCLUDING A BEVELED FIRST SURFACE PORTION AND A SECOND SURFACE PORTION LYING IN A PLANE NORMAL TO THE LONGITUDE AXIS OF SAID ENVELOPE, (D) SAID BEVELED FIRST PORTION AND AN ANNULAR PORTION OF SAID PLATE CO-EXTENSIVE WITH SAID BEVELED PORTION HAVING METALLIC COATINGS THEREON, (E) SAID SECOND PORTION BEING FREE FROM METALLIC COATING BONDED THERETO, AND 